ssh-ecdsa-sk.c

   1 /* $OpenBSD: ssh-ecdsa-sk.c,v 1.18 2023/03/08 04:43:12 guenther Exp $ */
   2 /*
   3  * Copyright (c) 2000 Markus Friedl.  All rights reserved.
   4  * Copyright (c) 2010 Damien Miller.  All rights reserved.
   5  * Copyright (c) 2019 Google Inc.  All rights reserved.
   6  *
   7  * Redistribution and use in source and binary forms, with or without
   8  * modification, are permitted provided that the following conditions
   9  * are met:
  10  * 1. Redistributions of source code must retain the above copyright
  11  *    notice, this list of conditions and the following disclaimer.
  12  * 2. Redistributions in binary form must reproduce the above copyright
  13  *    notice, this list of conditions and the following disclaimer in the
  14  *    documentation and/or other materials provided with the distribution.
  15  *
  16  * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
  17  * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
  18  * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
  19  * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
  20  * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
  21  * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
  22  * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
  23  * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
  24  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
  25  * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  26  */
  27
  28 /* #define DEBUG_SK 1 */
  29
  30 #include "includes.h"
  31
  32 #include <sys/types.h>
  33
  34 #ifdef WITH_OPENSSL
  35 #include <openssl/bn.h>
  36 #include <openssl/ec.h>
  37 #include <openssl/ecdsa.h>
  38 #include <openssl/evp.h>
  39 #endif
  40
  41 #include <string.h>
  42 #include <stdio.h> /* needed for DEBUG_SK only */
  43
  44 #include "openbsd-compat/openssl-compat.h"
  45
  46 #include "sshbuf.h"
  47 #include "ssherr.h"
  48 #include "digest.h"
  49 #define SSHKEY_INTERNAL
  50 #include "sshkey.h"
  51
  52 #ifndef OPENSSL_HAS_ECC
  53 /* ARGSUSED */
  54 int
  55 ssh_ecdsa_sk_verify(const struct sshkey *key,
  56     const u_char *signature, size_t signaturelen,
  57     const u_char *data, size_t datalen, u_int compat,
  58     struct sshkey_sig_details **detailsp)
  59 {
  60         return SSH_ERR_FEATURE_UNSUPPORTED;
  61 }
  62 #else /* OPENSSL_HAS_ECC */
  63
  64 /* Reuse some ECDSA internals */
  65 extern struct sshkey_impl_funcs sshkey_ecdsa_funcs;
  66
  67 static void
  68 ssh_ecdsa_sk_cleanup(struct sshkey *k)
  69 {
  70         sshkey_sk_cleanup(k);
  71         sshkey_ecdsa_funcs.cleanup(k);
  72 }
  73
  74 static int
  75 ssh_ecdsa_sk_equal(const struct sshkey *a, const struct sshkey *b)
  76 {
  77         if (!sshkey_sk_fields_equal(a, b))
  78                 return 0;
  79         if (!sshkey_ecdsa_funcs.equal(a, b))
  80                 return 0;
  81         return 1;
  82 }
  83
  84 static int
  85 ssh_ecdsa_sk_serialize_public(const struct sshkey *key, struct sshbuf *b,
  86     enum sshkey_serialize_rep opts)
  87 {
  88         int r;
  89
  90         if ((r = sshkey_ecdsa_funcs.serialize_public(key, b, opts)) != 0)
  91                 return r;
  92         if ((r = sshkey_serialize_sk(key, b)) != 0)
  93                 return r;
  94
  95         return 0;
  96 }
  97
  98 static int
  99 ssh_ecdsa_sk_serialize_private(const struct sshkey *key, struct sshbuf *b,
 100     enum sshkey_serialize_rep opts)
 101 {
 102         int r;
 103
 104         if (!sshkey_is_cert(key)) {
 105                 if ((r = sshkey_ecdsa_funcs.serialize_public(key,
 106                     b, opts)) != 0)
 107                         return r;
 108         }
 109         if ((r = sshkey_serialize_private_sk(key, b)) != 0)
 110                 return r;
 111
 112         return 0;
 113 }
 114
 115 static int
 116 ssh_ecdsa_sk_copy_public(const struct sshkey *from, struct sshkey *to)
 117 {
 118         int r;
 119
 120         if ((r = sshkey_ecdsa_funcs.copy_public(from, to)) != 0)
 121                 return r;
 122         if ((r = sshkey_copy_public_sk(from, to)) != 0)
 123                 return r;
 124         return 0;
 125 }
 126
 127 static int
 128 ssh_ecdsa_sk_deserialize_public(const char *ktype, struct sshbuf *b,
 129     struct sshkey *key)
 130 {
 131         int r;
 132
 133         if ((r = sshkey_ecdsa_funcs.deserialize_public(ktype, b, key)) != 0)
 134                 return r;
 135         if ((r = sshkey_deserialize_sk(b, key)) != 0)
 136                 return r;
 137         return 0;
 138 }
 139
 140 static int
 141 ssh_ecdsa_sk_deserialize_private(const char *ktype, struct sshbuf *b,
 142     struct sshkey *key)
 143 {
 144         int r;
 145
 146         if (!sshkey_is_cert(key)) {
 147                 if ((r = sshkey_ecdsa_funcs.deserialize_public(ktype,
 148                     b, key)) != 0)
 149                         return r;
 150         }
 151         if ((r = sshkey_private_deserialize_sk(b, key)) != 0)
 152                 return r;
 153
 154         return 0;
 155 }
 156
 157 /*
 158  * Check FIDO/W3C webauthn signatures clientData field against the expected
 159  * format and prepare a hash of it for use in signature verification.
 160  *
 161  * webauthn signatures do not sign the hash of the message directly, but
 162  * instead sign a JSON-like "clientData" wrapper structure that contains the
 163  * message hash along with a other information.
 164  *
 165  * Fortunately this structure has a fixed format so it is possible to verify
 166  * that the hash of the signed message is present within the clientData
 167  * structure without needing to implement any JSON parsing.
 168  */
 169 static int
 170 webauthn_check_prepare_hash(const u_char *data, size_t datalen,
 171     const char *origin, const struct sshbuf *wrapper,
 172     uint8_t flags, const struct sshbuf *extensions,
 173     u_char *msghash, size_t msghashlen)
 174 {
 175         int r = SSH_ERR_INTERNAL_ERROR;
 176         struct sshbuf *chall = NULL, *m = NULL;
 177
 178         if ((m = sshbuf_new()) == NULL ||
 179             (chall = sshbuf_from(data, datalen)) == NULL) {
 180                 r = SSH_ERR_ALLOC_FAIL;
 181                 goto out;
 182         }
 183         /*
 184          * Ensure origin contains no quote character and that the flags are
 185          * consistent with what we received
 186          */
 187         if (strchr(origin, '\"') != NULL ||
 188             (flags & 0x40) != 0 /* AD */ ||
 189             ((flags & 0x80) == 0 /* ED */) != (sshbuf_len(extensions) == 0)) {
 190                 r = SSH_ERR_INVALID_FORMAT;
 191                 goto out;
 192         }
 193
 194         /*
 195          * Prepare the preamble to clientData that we expect, poking the
 196          * challenge and origin into their canonical positions in the
 197          * structure. The crossOrigin flag and any additional extension
 198          * fields present are ignored.
 199          */
 200 #define WEBAUTHN_0      "{\"type\":\"webauthn.get\",\"challenge\":\""
 201 #define WEBAUTHN_1      "\",\"origin\":\""
 202 #define WEBAUTHN_2      "\""
 203         if ((r = sshbuf_put(m, WEBAUTHN_0, sizeof(WEBAUTHN_0) - 1)) != 0 ||
 204             (r = sshbuf_dtourlb64(chall, m, 0)) != 0 ||
 205             (r = sshbuf_put(m, WEBAUTHN_1, sizeof(WEBAUTHN_1) - 1)) != 0 ||
 206             (r = sshbuf_put(m, origin, strlen(origin))) != 0 ||
 207             (r = sshbuf_put(m, WEBAUTHN_2, sizeof(WEBAUTHN_2) - 1)) != 0)
 208                 goto out;
 209 #ifdef DEBUG_SK
 210         fprintf(stderr, "%s: received origin: %s\n", __func__, origin);
 211         fprintf(stderr, "%s: received clientData:\n", __func__);
 212         sshbuf_dump(wrapper, stderr);
 213         fprintf(stderr, "%s: expected clientData premable:\n", __func__);
 214         sshbuf_dump(m, stderr);
 215 #endif
 216         /* Check that the supplied clientData has the preamble we expect */
 217         if ((r = sshbuf_cmp(wrapper, 0, sshbuf_ptr(m), sshbuf_len(m))) != 0)
 218                 goto out;
 219
 220         /* Prepare hash of clientData */
 221         if ((r = ssh_digest_buffer(SSH_DIGEST_SHA256, wrapper,
 222             msghash, msghashlen)) != 0)
 223                 goto out;
 224
 225         /* success */
 226         r = 0;
 227  out:
 228         sshbuf_free(chall);
 229         sshbuf_free(m);
 230         return r;
 231 }
 232
 233 static int
 234 ssh_ecdsa_sk_verify(const struct sshkey *key,
 235     const u_char *sig, size_t siglen,
 236     const u_char *data, size_t dlen, const char *alg, u_int compat,
 237     struct sshkey_sig_details **detailsp)
 238 {
 239         ECDSA_SIG *esig = NULL;
 240         BIGNUM *sig_r = NULL, *sig_s = NULL;
 241         u_char sig_flags;
 242         u_char msghash[32], apphash[32], sighash[32];
 243         u_int sig_counter;
 244         int is_webauthn = 0, ret = SSH_ERR_INTERNAL_ERROR;
 245         struct sshbuf *b = NULL, *sigbuf = NULL, *original_signed = NULL;
 246         struct sshbuf *webauthn_wrapper = NULL, *webauthn_exts = NULL;
 247         char *ktype = NULL, *webauthn_origin = NULL;
 248         struct sshkey_sig_details *details = NULL;
 249 #ifdef DEBUG_SK
 250         char *tmp = NULL;
 251 #endif
 252
 253         if (detailsp != NULL)
 254                 *detailsp = NULL;
 255         if (key == NULL || key->ecdsa == NULL ||
 256             sshkey_type_plain(key->type) != KEY_ECDSA_SK ||
 257             sig == NULL || siglen == 0)
 258                 return SSH_ERR_INVALID_ARGUMENT;
 259
 260         if (key->ecdsa_nid != NID_X9_62_prime256v1)
 261                 return SSH_ERR_INTERNAL_ERROR;
 262
 263         /* fetch signature */
 264         if ((b = sshbuf_from(sig, siglen)) == NULL)
 265                 return SSH_ERR_ALLOC_FAIL;
 266         if ((details = calloc(1, sizeof(*details))) == NULL) {
 267                 ret = SSH_ERR_ALLOC_FAIL;
 268                 goto out;
 269         }
 270         if (sshbuf_get_cstring(b, &ktype, NULL) != 0) {
 271                 ret = SSH_ERR_INVALID_FORMAT;
 272                 goto out;
 273         }
 274         if (strcmp(ktype, "webauthn-sk-ecdsa-sha2-nistp256@openssh.com") == 0)
 275                 is_webauthn = 1;
 276         else if (strcmp(ktype, "sk-ecdsa-sha2-nistp256@openssh.com") != 0) {
 277                 ret = SSH_ERR_INVALID_FORMAT;
 278                 goto out;
 279         }
 280         if (sshbuf_froms(b, &sigbuf) != 0 ||
 281             sshbuf_get_u8(b, &sig_flags) != 0 ||
 282             sshbuf_get_u32(b, &sig_counter) != 0) {
 283                 ret = SSH_ERR_INVALID_FORMAT;
 284                 goto out;
 285         }
 286         if (is_webauthn) {
 287                 if (sshbuf_get_cstring(b, &webauthn_origin, NULL) != 0 ||
 288                     sshbuf_froms(b, &webauthn_wrapper) != 0 ||
 289                     sshbuf_froms(b, &webauthn_exts) != 0) {
 290                         ret = SSH_ERR_INVALID_FORMAT;
 291                         goto out;
 292                 }
 293         }
 294         if (sshbuf_len(b) != 0) {
 295                 ret = SSH_ERR_UNEXPECTED_TRAILING_DATA;
 296                 goto out;
 297         }
 298
 299         /* parse signature */
 300         if (sshbuf_get_bignum2(sigbuf, &sig_r) != 0 ||
 301             sshbuf_get_bignum2(sigbuf, &sig_s) != 0) {
 302                 ret = SSH_ERR_INVALID_FORMAT;
 303                 goto out;
 304         }
 305         if (sshbuf_len(sigbuf) != 0) {
 306                 ret = SSH_ERR_UNEXPECTED_TRAILING_DATA;
 307                 goto out;
 308         }
 309
 310 #ifdef DEBUG_SK
 311         fprintf(stderr, "%s: data: (len %zu)\n", __func__, datalen);
 312         /* sshbuf_dump_data(data, datalen, stderr); */
 313         fprintf(stderr, "%s: sig_r: %s\n", __func__, (tmp = BN_bn2hex(sig_r)));
 314         free(tmp);
 315         fprintf(stderr, "%s: sig_s: %s\n", __func__, (tmp = BN_bn2hex(sig_s)));
 316         free(tmp);
 317         fprintf(stderr, "%s: sig_flags = 0x%02x, sig_counter = %u\n",
 318             __func__, sig_flags, sig_counter);
 319         if (is_webauthn) {
 320                 fprintf(stderr, "%s: webauthn origin: %s\n", __func__,
 321                     webauthn_origin);
 322                 fprintf(stderr, "%s: webauthn_wrapper:\n", __func__);
 323                 sshbuf_dump(webauthn_wrapper, stderr);
 324         }
 325 #endif
 326         if ((esig = ECDSA_SIG_new()) == NULL) {
 327                 ret = SSH_ERR_ALLOC_FAIL;
 328                 goto out;
 329         }
 330         if (!ECDSA_SIG_set0(esig, sig_r, sig_s)) {
 331                 ret = SSH_ERR_LIBCRYPTO_ERROR;
 332                 goto out;
 333         }
 334         sig_r = sig_s = NULL; /* transferred */
 335
 336         /* Reconstruct data that was supposedly signed */
 337         if ((original_signed = sshbuf_new()) == NULL) {
 338                 ret = SSH_ERR_ALLOC_FAIL;
 339                 goto out;
 340         }
 341         if (is_webauthn) {
 342                 if ((ret = webauthn_check_prepare_hash(data, dlen,
 343                     webauthn_origin, webauthn_wrapper, sig_flags, webauthn_exts,
 344                     msghash, sizeof(msghash))) != 0)
 345                         goto out;
 346         } else if ((ret = ssh_digest_memory(SSH_DIGEST_SHA256, data, dlen,
 347             msghash, sizeof(msghash))) != 0)
 348                 goto out;
 349         /* Application value is hashed before signature */
 350         if ((ret = ssh_digest_memory(SSH_DIGEST_SHA256, key->sk_application,
 351             strlen(key->sk_application), apphash, sizeof(apphash))) != 0)
 352                 goto out;
 353 #ifdef DEBUG_SK
 354         fprintf(stderr, "%s: hashed application:\n", __func__);
 355         sshbuf_dump_data(apphash, sizeof(apphash), stderr);
 356         fprintf(stderr, "%s: hashed message:\n", __func__);
 357         sshbuf_dump_data(msghash, sizeof(msghash), stderr);
 358 #endif
 359         if ((ret = sshbuf_put(original_signed,
 360             apphash, sizeof(apphash))) != 0 ||
 361             (ret = sshbuf_put_u8(original_signed, sig_flags)) != 0 ||
 362             (ret = sshbuf_put_u32(original_signed, sig_counter)) != 0 ||
 363             (ret = sshbuf_putb(original_signed, webauthn_exts)) != 0 ||
 364             (ret = sshbuf_put(original_signed, msghash, sizeof(msghash))) != 0)
 365                 goto out;
 366         /* Signature is over H(original_signed) */
 367         if ((ret = ssh_digest_buffer(SSH_DIGEST_SHA256, original_signed,
 368             sighash, sizeof(sighash))) != 0)
 369                 goto out;
 370         details->sk_counter = sig_counter;
 371         details->sk_flags = sig_flags;
 372 #ifdef DEBUG_SK
 373         fprintf(stderr, "%s: signed buf:\n", __func__);
 374         sshbuf_dump(original_signed, stderr);
 375         fprintf(stderr, "%s: signed hash:\n", __func__);
 376         sshbuf_dump_data(sighash, sizeof(sighash), stderr);
 377 #endif
 378
 379         /* Verify it */
 380         switch (ECDSA_do_verify(sighash, sizeof(sighash), esig, key->ecdsa)) {
 381         case 1:
 382                 ret = 0;
 383                 break;
 384         case 0:
 385                 ret = SSH_ERR_SIGNATURE_INVALID;
 386                 goto out;
 387         default:
 388                 ret = SSH_ERR_LIBCRYPTO_ERROR;
 389                 goto out;
 390         }
 391         /* success */
 392         if (detailsp != NULL) {
 393                 *detailsp = details;
 394                 details = NULL;
 395         }
 396  out:
 397         explicit_bzero(&sig_flags, sizeof(sig_flags));
 398         explicit_bzero(&sig_counter, sizeof(sig_counter));
 399         explicit_bzero(msghash, sizeof(msghash));
 400         explicit_bzero(sighash, sizeof(msghash));
 401         explicit_bzero(apphash, sizeof(apphash));
 402         sshkey_sig_details_free(details);
 403         sshbuf_free(webauthn_wrapper);
 404         sshbuf_free(webauthn_exts);
 405         free(webauthn_origin);
 406         sshbuf_free(original_signed);
 407         sshbuf_free(sigbuf);
 408         sshbuf_free(b);
 409         ECDSA_SIG_free(esig);
 410         BN_clear_free(sig_r);
 411         BN_clear_free(sig_s);
 412         free(ktype);
 413         return ret;
 414 }
 415
 416 static const struct sshkey_impl_funcs sshkey_ecdsa_sk_funcs = {
 417         /* .size = */           NULL,
 418         /* .alloc = */          NULL,
 419         /* .cleanup = */        ssh_ecdsa_sk_cleanup,
 420         /* .equal = */          ssh_ecdsa_sk_equal,
 421         /* .ssh_serialize_public = */ ssh_ecdsa_sk_serialize_public,
 422         /* .ssh_deserialize_public = */ ssh_ecdsa_sk_deserialize_public,
 423         /* .ssh_serialize_private = */ ssh_ecdsa_sk_serialize_private,
 424         /* .ssh_deserialize_private = */ ssh_ecdsa_sk_deserialize_private,
 425         /* .generate = */       NULL,
 426         /* .copy_public = */    ssh_ecdsa_sk_copy_public,
 427         /* .sign = */           NULL,
 428         /* .verify = */         ssh_ecdsa_sk_verify,
 429 };
 430
 431 const struct sshkey_impl sshkey_ecdsa_sk_impl = {
 432         /* .name = */           "sk-ecdsa-sha2-nistp256@openssh.com",
 433         /* .shortname = */      "ECDSA-SK",
 434         /* .sigalg = */         NULL,
 435         /* .type = */           KEY_ECDSA_SK,
 436         /* .nid = */            NID_X9_62_prime256v1,
 437         /* .cert = */           0,
 438         /* .sigonly = */        0,
 439         /* .keybits = */        256,
 440         /* .funcs = */          &sshkey_ecdsa_sk_funcs,
 441 };
 442
 443 const struct sshkey_impl sshkey_ecdsa_sk_cert_impl = {
 444         /* .name = */           "sk-ecdsa-sha2-nistp256-cert-v01@openssh.com",
 445         /* .shortname = */      "ECDSA-SK-CERT",
 446         /* .sigalg = */         NULL,
 447         /* .type = */           KEY_ECDSA_SK_CERT,
 448         /* .nid = */            NID_X9_62_prime256v1,
 449         /* .cert = */           1,
 450         /* .sigonly = */        0,
 451         /* .keybits = */        256,
 452         /* .funcs = */          &sshkey_ecdsa_sk_funcs,
 453 };
 454
 455 const struct sshkey_impl sshkey_ecdsa_sk_webauthn_impl = {
 456         /* .name = */           "webauthn-sk-ecdsa-sha2-nistp256@openssh.com",
 457         /* .shortname = */      "ECDSA-SK",
 458         /* .sigalg = */         NULL,
 459         /* .type = */           KEY_ECDSA_SK,
 460         /* .nid = */            NID_X9_62_prime256v1,
 461         /* .cert = */           0,
 462         /* .sigonly = */        1,
 463         /* .keybits = */        256,
 464         /* .funcs = */          &sshkey_ecdsa_sk_funcs,
 465 };
 466
 467 #endif /* OPENSSL_HAS_ECC */